When used in the context of being one of the SixThinkingHats, the BlackHat represents the down side, negative consequences, the worst things that could happen.
Or just used as a temporary name (local constant) to help distinguish between multiple semi-anonymous authors.
The term is more often used to describe villains, including computer crackers. Often the term is used to refer to a hacker/cracker who seeks to a) commit espionage (discover confidential information), b) commit fraud , or c) engage in vandalism (or worse).
This negative usage has nothing to do with SixThinkingHats.
See xkcd: http://xkcd.com/374/ and http://comicjk.com/comic.php/173 for details.
This has nothing to do with the SortingHat?. -- EricJablow(Does the term SortingHat? have any meaning in comp sci, or is it just a reference to the HarryPotter novels???)